1. Field of the Invention
The present invention relates to a fusing apparatus, and more particularly, to a fusing apparatus for fusing a toner picture image onto a printing medium.
2. Description of the Related Art
A fusing apparatus (utilized in, for example, an electrophotographic printer or a copier) employs a rotatable heater roller, a rotatable back-up roller pressed to the heater roller and a plurality of movable separation claws attached in a direction parallel to the axis of the heater roller. The front end of each of the separation claws is pressed by an actuating member such as a compression spring to the peripheral surface of the heater roller. The position where the front end of a separation claw is pressed to the heater roller is located a little downstream from the position where the heater roller is pressed to the back-up roller.
When paper is transferred between the heater roller and the back-up roller, unfused toner adhered to the paper is melted and becomes highly viscous due to the heat of the heater roller and therefore fuses to the paper. When the paper is transferred to the position where the front ends of the separation claws are pressed to the heater roller, with the paper being stuck to the heater roller due to the viscosity of the toner, the paper is stripped off from the heater roller by the separation claws.
When the paper stuck to the heater roller is stripped off by the separation claws in the conventional fusing apparatus, the paper has been curled, since the curvature of the heater roller is preserved in the paper after stripping due to the adhesion of the paper to the heater roller. The conventional fusing apparatus is not equipped with correction members to reform the curl of the paper into the original state, so it is difficult to stack up plural sheets of paper in alignment with each other after printing, because the paper retains the curled state without returning to the original state when the toner is cooled down and solidified. Moreover, the conditions for stacking paper with a paper stacker become aggravated. Further, problems may arise during double faced printing. When a curled paper, one face of which is already printed but not the other face, is retransmitted to a printing unit for printing on the other face, paper feeding may be hindered in the paper transfer paths, which causes paper jams. Accordingly, the adhesion of the paper to the heater roller has been prevented by adjusting the cohesive force (Ft) of the toner to be larger than the adhesive force (Frt) of the toner to the heater roller, and by adjusting the adhesive force (Ftp) of the toner to the paper to be larger than the cohesive force (Ft), that is, Frt&lt;Ft&lt;Ftp. However, it has not previously been possible to strip the paper completely from the heater roller regardless of the paper utilized and to prevent the curl resulting from the application of heating or pressure due to the fusing conditions (temperature, pressure and heating time), toner ingredients (degradation of toner) or the lapse of time.
The above described technology is disclosed, for example, in Japanese Patent Laid-open Publication No. 59-34572.